For the hottest month of the year at the proposed airport site, the monthly mean of the average daily temperature is $39^{\circ}C$. The monthly mean of the maximum daily temperature is $48^{\circ}C$ for the same month of the year. From the given information, the calculated Airport Reference Temperature $\text{(in}\: ^{\circ}C),$ is
- $36$
- $39$
- $42$
- $48$